The formation of mature viruses is an important research area, not only because of what we learn about the viruses themselves, but because viruses are simple model systems for elucidating the fundamental principles behind the assembly of large macromolecular complexes. This proposal presents a combined modeling and experimental approach to the investigation of two kinds of small, unenveloped, icosahedral viruses. Bacteriophages package their double-stranded DNA genomes into preformed protein capsids, using an ATP-driven motor to overcome the electrostatic, elastic and entropic forces that oppose packaging. In contrast, single-stranded RNA viruses assemble spontaneously, with the protein capsid forming around the outside of the RNA genome without the expenditure of energy. We will continue our investigations of the structural and thermodynamic aspects of DNA packaging in bacteriophage, focusing our efforts on DNA-DNA and DNA-capsid interactions, and on DNA kinking. We will also undertake an investigation of the forces opposing the compaction of DNA to high density in non-viral systems. The principles learned from these efforts will have implications for the understanding of genome structures and the regulation of gene expression in both prokaryotic and eukaryotic cells. Our investigations on RNA viruses will include the develop of improved three-dimensional models for satellite tobacco mosaic virus (STMV) and pariacoto virus (PaV);the development of methods for modeling assembly of RNA viruses;and experimental investigations of the condensation of RNA by polycations, particularly basic polypeptides. The work on RNA viruses is based on the hypothesis that assembly is initiated by strong but nonspecific interactions between positively charged polypeptides and the negatively charged RNA, leading to a collapsed aggregate resembling condensed DNA, followed by the formation of the mature virus. If correct, this hypothesis could provide the basis for new drug design strategies. The results might well apply to the formation of other large RNA-protein assemblies as well. PUBLIC HEALTH RELEVANCE Viruses cause many human diseases. An understanding of how they are assembled would offer important opportunities for the development of new antiviral drugs. In addition, bacteriophage (viruses that infect bacteria) have been used occasionally in the past to treat bacterial infections in humans. Knowledge about bacteriophage assembly could prove useful in the design of new antibacterial agents.